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I don't think you're missing anything, that does seem like a loophole. It exists because two rules exist for different reasons.

Users with < 10 rep are limited in the number of links (and images) they can post in one answer. You're right, this is to cut back on spam.

You can always comment on your own posts. This helps cut back on the number of new users who post follow-up comments as answers. Commenting on your own posts is so commonly needed that it makes sense to let everyone do it. (You can only pollute your own posts with unnecessary comments when you have < 50 rep, so at least it's contained.)

I'm not sure if anyone has actually exploited this loophole to post spam. Flag it if you see it. If it gets to be a problem the loophole might get closed.

Indeed. I imagine that there's enough barrier in having to post a question, know that you can put multiple (or any) links in comments, and actually doing so that there's not a lot of upfront value for performing that check.
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Tim StoneJan 10 '12 at 20:28